The production, processing, and recycling of aluminum commonly requires melting aluminum stock, such as scrap stock, that has a high surface area to weight ratio (also referred to as light gauge stock). Examples of such material include scalper chips generated from milling rolling ingots, turnings or swarf from lathe or sawing operations, and output from crushers or shredders used to recycle aluminum sheet, extrusions, or cast shapes. The material is typically melted in a melting chamber of a melting furnace, also referred to as a furnace system, such as a rotary furnace or a reverb furnace. It is desirable to submerge solid aluminum pieces quickly in a molten metal bath to reduce oxidation of the aluminum caused by contact of aluminum surfaces with air and accelerated by high temperatures. One conventional technique for submerging the aluminum scrap includes pushing the light gauge scrap under the surface of the molten metal. Another technique includes creating flow to a melting well to drag scrap under the surface of the molten metal bath.
Light gauge aluminum scrap is intrinsically difficult to efficiently melt because aluminum and its alloys oxidize very readily. The process of oxidation turns the surface from aluminum, a valuable metal, to aluminum oxide, a non-metallic mineral of comparatively lower value. Unprotected aluminum surfaces oxidize rapidly in air, even at ambient temperatures. When exposed to temperatures high enough to melt aluminum, the oxidation process is greatly accelerated. Thus, if aluminum is exposed to air at such temperatures, oxidation can completely consume aluminum having thin cross sections, that is, aluminum with a high surface area to weight ratio.
One historical technique for melting light gauge scrap is to use a rotary salt bath furnace. Rotary furnaces are capable of melting a wide variety of scrap types. Due to operational and environmental concerns, this technique is increasingly focused on processing aluminum dross.
Rotary furnaces are progressively being replaced by aluminum reverberatory furnace systems, also referred to as reverb furnaces or reverb furnace systems, for melting light gauge scrap. A typical reverb furnace system includes a heated hearth and a side well subsystem, which may include one or more side wells, such as a pumping well, a melting well, a dross skimming well, etc. When using a reverb furnace for scrap melting, it is standard practice to leave a molten metal “heel” in the furnace, and to use this reservoir of metal as the proximate heat source for melting the scrap. Heavier gauge scrap tends to quickly submerge in the molten metal and melt using the superheat in the molten metal. The molten metal bath effectively excludes air, which enables submerged melting to occur without excessive oxidation. This operation usually takes place not in the heated hearth of the reverb furnace, but in a side well located adjacent the hearth, which communicates with the hearth by means of submerged arches. In contrast to the heavier gauge scrap, light gauge scrap tends to float rather than sink. Thus, a technique or subsystem is sometimes employed to submerge the light gauge scrap into a side well bath of the side well subsystem to melt the scrap. An exemplary technique is to submerge the scrap using a turbulent flow in a melting well.
Because only the hearth is typically heated, the side well bath can quickly be chilled to the relative freezing point in the localized melting zone if the heat is not replaced by hot molten metal continuously circulating into and out of the melting zone. An inducer disposed upstream of a melting well where melting takes place is often used to circulate hot molten metal into the melting well.
An exemplary conventional system of this nature, that has been put into commercial use, is generally disclosed in U.S. Pat. No. 4,286,985. Another commercialized version of this type of system has a pump that supplies molten metal to a melting well via a single enclosed molten metal flow path. These types of conventional configurations and other similar systems all rely on a differential head or the column of molten metal vertically disposed between the elevated metal level in the melting well and the current metal level in the hearth and/or a pumping well to force molten metal downstream of the melting well. Therefore, the differential head dictates flow of molten metal out of the melting well, generally without regard to the physical configuration of the melting well.